1. Field of the Invention
The present invention relates to an aqueous coating liquid suitable for forming a magnesium oxide film used as a dielectric protective film in a plasma display panel (PDP) or the like by the coating pyrolysis method.
2. Description of the Related Art
Magnesium oxide films are used in a wide range of applications such as protective films for PDPs, insulating films, catalyst films, or surface protective films. Conventionally, films of metal oxide such as films of magnesium oxide have been formed by a physical film-forming method such as sputtering and vacuum evaporation. When these methods are employed, a uniform, dense and highly crystalline film can be obtained. On the other hand, since the films are formed under a vacuum, a large, complicated and expensive apparatus is necessary. Furthermore, since the films are produced by batch production, the production efficiency is low and the production cost is high.
In contrast to physical methods, the coating pyrolysis method may be employed by which a metal oxide film can be formed in a simple process without an expensive and complex apparatus. This coating pyrolysis method includes a process of applying a coating liquid containing a desired metal compound on a substrate uniformly, a process of removing a solvent in the resultant film completely by pre-baking, and a process of performing baking at a high temperature. In order to obtain a uniform and transparent metal oxide film as a final product, it is necessary to obtain a good coating properties and to maintain the uniformity and the transparency of the formed film in each process.
Japanese Laid-Open Patent Publication Nos. 9-95627 and 9-129141 disclose a method for forming a magnesium oxide film using a coating liquid in which a magnesium carboxylate is dissolved in a solvent. All of the solvents used in these methods are organic solvents such as xylene or butyl carbitol. When a film is formed with a coating liquid using such an organic solvent, uniform coating can be easily performed on a substrate, but at pre-baking after the substrate is coated, the organic solvent is released in the air, which may harm the environment.
On the other hand, when a film is formed by coating using a solution in which a metal compound such as a water soluble magnesium carboxylate is contained in a water-based solvent, no organic solvent is released, so that the adverse effect to the environment can be reduced. However, when a substrate is coated with a coating liquid containing a water-based solvent, the coating liquid is repelled on the substrate by the surface tension of the water, so that no uniform film can be obtained. The water soluble magnesium carboxylate is generally derived from a short chain carboxylic acid, and therefore the crystallinity of the magnesium carboxylate itself is high. For this reason, when the substrate is coated with such a coating liquid and pre-baked, the magnesium carboxylate crystallizes and is precipitated, and thus it is difficult to obtain a transparent and uniform film on the substrate.
As a method for coating a substrate with an aqueous coating liquid, Japanese Laid-Open Patent Publication No. 2001-10816 discloses a coating liquid that comprises an aqueous solution containing a metal complex in which a metal such as copper, titanium, nickel, or magnesium is coordinated to a ligand such as EDTA, or a salt thereof, and a water-soluble polymer such as polyethylene glycol, polyvinyl alcohol, and collagen as a film-forming substance. However, although the substrate can be coated uniformly with such a coating liquid containing the above-mentioned polymer, opaqueness occurs when pre-baking is performed, and thus it is difficult to form a film having excellent uniformity and transparency.